As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for batteries has also sharply increased as an energy source for the mobile devices. Also, much research on batteries satisfying various needs has been carried out.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries, which are thin enough to be applied to products, such as mobile phones, is very high. In terms of the material for batteries, on the other hand, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, having high energy density, high discharge voltage, and high output stability, is very high.
Furthermore, secondary batteries may be classified based on the construction of an electrode assembly having a cathode/separator/anode structure. For example, the electrode assembly may be constructed in a jelly-roll (winding) type structure in which long-sheet type cathodes and long-sheet type anodes are wound while separators are disposed respectively between the cathodes and the anodes, a stacking type structure in which pluralities of cathodes and anodes having a predetermined size are successively stacked while separators are disposed respectively between the cathodes and the anodes, or a stacking/folding type structure in which pluralities of cathodes and anodes having a predetermined size are successively stacked while separators are disposed respectively between the cathodes and the anodes to constitute a bi-cell or a full-cell, and then the bi-cell or the full-cell is wound.
FIG. 1 is a side view typically illustrating the general structure of a conventional representative stacking type electrode assembly.
Referring to FIG. 1, the stacking type electrode assembly 10 is constructed in a structure in which cathodes 20, each of which has a cathode active material 22 applied to the opposite major surfaces of a cathode current collector 21, and anodes 30, each of which has an anode active material 32 applied to the opposite major surfaces of an anode current collector 31, are sequentially stacked while separators 70 are disposed respectively between the cathodes 20 and the anodes 30.
From one-side ends of the cathode current collectors 21 and the anode current collectors 31 protrude pluralities of cathode tabs 41 and anode tabs 51, to which an active material is not applied, such that the cathode tabs 41 and the anode tabs 51 are electrically connected to a cathode lead 60 and an anode lead (not shown) constituting electrode terminals of a battery (not shown). The cathode tabs 41 and the anode tabs 51 are joined in a concentrated state, and are then connected to the cathode lead 60 and the anode lead, respectively. This structure is more clearly illustrated in FIG. 2, which is a partially enlarged view typically illustrating the joint portion between the cathode tabs and the cathode lead. FIG. 2 illustrates only the joint portion between the cathode tabs and the cathode lead for convenience of description, although this structure is also applied to the joint portion between the anode tabs and the anode lead.
Referring to FIG. 2, the cathode tabs 40 are brought into tight contact with each other in the direction indicated by an arrow, and are connected to the cathode lead 60. Specifically, the cathode tabs 40 are brought into tight contact with the cathode lead 60 adjacent to the lowermost cathode tab 42 such that the cathode tabs 40 are joined to each other with the smallest bending length. Consequently, the length difference occurs at a joint portion A of the cathode tabs 40 between the lowermost cathode tab 42, which is a short distance from the cathode lead 60, and the uppermost cathode tab 41, which is a long distance from the cathode lead 60, due to the distance difference between the cathode tabs 40 and the cathode lead 60. Of course, this length difference also occurs at a joint portion between the anode tabs and the anode lead.
Due to this structure, the areas where the electrode tabs are in contact with the electrode lead are different from each other at the joint portion between the electrode tabs and the electrode lead of the electrode assembly, with the result that the use of electrode leads having more than a necessary size is inevitable. This is because the size of the electrode lead is set based on the electrode tab having the largest contact area so as to fix the electrode tabs, the thickness of which is much less than the electrode lead. In addition, the structural stability of the electrode assembly having the above-described structure is lowered for this reason.
In order to solve the above-mentioned problem, there has been proposed a technology for joining electrode tabs, while the electrode tabs are in tight contact with each other in one direction, and uniformly cutting the electrode tabs using a cutter. When using this technology, however, burrs occur at the cut ends of the electrode tabs, when the electrode tabs are cut. In addition, it is necessary to additionally perform a cutting process.
On the other hand, it is possible to bend the end of the electrode lead to a predetermined angle, the short-distance electrode tab to a large angle, and the long-distance electrode tab to a small angle in order to reduce the length difference between the electrode tabs at the joint portion thereof. In this structure, however, the size of the joint portion between the electrode tabs and the electrode lead is increased due to the bending, with the result that the electrode assembly may move, when external impacts are applied to the electrode assembly, and therefore, a possibility of the occurrence of a short circuit is strong. Especially for a middle- or large-sized battery cell, the thickness of the electrode lead is approximately twice that of a small-sized battery cell, with the result that it is more difficult to apply the above-described structure.
Consequently, there is a high necessity for an electrode assembly having an improved structure in which the lengths of the electrode tabs at the joint portion thereof are the same while the bending of the electrode tabs is minimized.